A PIN photodiode is an element that has a P-I-N structure including an Intrinsic layer (high-resistance epitaxial layer or the like) between a p-type semiconductor and an n-type semiconductor, and it converts the incident light into photocurrent. It has the following theory. When light having higher energy than the energy band gap is incident into silicon (Si) having an inversely biased applied PIN structure, electron/hole pairs are generated in the silicon crystal. With them acting as light carriers, the electrons move to the n layer, while the holes move to the p layer. As a result, a current is output in the reverse direction. An example of a conventional PIN device is Japanese Patent Application No. 2001-320079.